1. Field of the Invention
The present invention relates to a device for positioning a flattening filter in the center of an x-ray radiation. In particular, it relates to a device for positioning a flattening filter in the center of an x-ray radiation produced by an electron accelerator for radiation therapy due to a deceleration of the electrons in a so-called target.
2. Description of the Prior Art
An electron accelerator for radiation therapy is for example described in the essay "Radiotherapy today: The Mevatron 20, a Compact Highoutput Linear Accelerator", by W. Haas, V. Stieber and L. Taumann in Electromedica 3-4/77, pages 101-106. Such an electron accelerator is also specified in the Siemens brochure "Siemens, a total resource company for radiation therapy", MG/5020-008 SIQ 785.
In the case of electron accelerators x-ray deceleration radiation is produced due to a deceleration of the electrons in a so-called target. It is known in the art to balance or compensate the dosage in a given space angle range of the x-rays leaving the target by placing a so-called flattening filter into the portion of the x-ray cone of interest. This flattening filter has a conical design and its contour path is adapted to the path of the radiation intensity at the place of use. Special kinds of flattening filters are for example described in the U.S. Pat. Nos. 4,109,154 (Taumann), 4,121,109 (Taumann et al.) and 4,343,997 (Heinz).
As indicated in column 1, lines 20-25 of Taumann's U.S. Pat. No. 4,109,154 the tip of a flattening filter must be positioned very precisely with respect to the center beam of the x-ray radiation.